This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The clam, Scapharca inaequivalvis, possesses two hemoglobins that represent exceptional model systems for the investigation of protein allostery. Both hemoglobins bind oxygen cooperatively using a structural mechanism that is very different from the more well studied human hemoglobin. The dimeric hemoglobin, termed HbI, is the simplest possible model system for allostery with two identical subunits. Time-resolved crystallographic analysis of this hemoglobin provided, for the first time, a preliminary structural description of allosteric changes in real time (Knapp et. al. 2006, PNAS 103 7649-7654). Despite the overall success of these experiments, a major drawback was the very high level of geminate rebinding in the crystal, which substantially reduced the signal during the allosteric transition. Our analysis of ligand migration, including time-resolved crystallographic experiments, solution experiments and computational analysis (Knapp et al. 2009, Structure 17, in press) strongly suggests the crystal lattice restricts ligand exit by damping transient subunit rotations that are required for exit through a distal histidine gate. These experiments also revealed a potential alternate exit route through a "back door" channel. We are producing mutants that will allow ligands to exit through this back door within the tight confines of the crystal lattice. The tetrameric hemoglobin, termed HbII, is formed from two heterodimers, each of which has a similar assembly to that of HbI. The presence of two different subunits will permit investigation of how one subunit impacts a second subunit, which is not possible in the two-fold symmetric HbI. Therefore, we propose to use time-resolved x-ray diffraction experiments to elucidate the kinetic structural pathway in the tetrameric HbII and specific mutants of HbII.